1. Field of the Invention
The present invention relates to an encoder that is usable for various apparatuses such as optical apparatuses and outputs, with relative movement of a scale and a sensor, a signal showing position of a movable member provided in the apparatus.
2. Description of the Related Art
Encoders that are used to detect position and speed of a movable member are constituted by a scale provided with a periodic pattern and a sensor outputting a periodic signal that periodically changes corresponding to the periodic pattern with relative movement of the sensor and the scale. Such encoders include an incremental encoder that, for example, produces two periodic signals (two phase signals) having mutually different phases from one periodic pattern and detects a relative position of the scale and the sensor from number of periods and the phases of the two phase signals. Moreover, the encoders also include an absolute encoder that, for example, produces two paired two phase signals from two periodic patterns having mutually different periods and detects an absolute position of the scale or the sensor from a difference (phase difference) between the phases of one of the two paired two phase signals and the phases of the other paired two phase signals.
Such an absolute encoder has an advantage that a reset operation to detect a reference position for position detection before start of the position detection such as at a time of power-on, which is needed for the incremental encoder, is not needed.
Japanese Patent Laid-Open No. 2012-026765 discloses an absolute encoder capable of acquiring more information from a small number of scale tracks by forming multiple periodic patterns whose periods are mutually different in one scale track and by switching a detection period of a sensor in chronological order. The absolute encoder disclosed in Japanese Patent Laid-Open No. 2012-026765 requires, in order to read the periodic patterns having the mutually different periods in chronological order, switching of the detection period of the sensor according to the period of the periodic pattern to be read.
However, switching of the detection period of the sensor generally needs a sufficient time period though it depends on characteristics of the sensor and a circuit relating thereto. Moreover, since reading of the periodic pattern, that is, detection of a positional change cannot be performed during the switching of the detection period, a large relative positional change amount (high relative movement speed) of the scale and the sensor during the switching of the detection period generates a detection error in a detected position after the switching of the detection period.